The Universe was born in the Big Bang and almost instantly passed through a phase of inflation — an ultra-fast expansion that blew up microscopic regions to cosmological scales.
Light from a distant galaxy travels across the universe, passing through vast clumps of dark and ordinary matter. Their gravity slightly bends the path of the rays, like a river's current flowing around invisible boulders. In images, this shows up as a barely noticeable elongation of galaxy shapes, as if they were stretched on an invisible frame.